In magnetooptical disk memories, the data are known to be carried by magnetic disks and read by optoelectronic devices. The radial and longitudinal recording densities of these memories are on the order of 10,000 tracks per cm and 10,000 data per cm, respectively. (Radial density is the number of tracks per unit of length, measured along the diameter of the disk, while longitudinal density is the number of data per unit of length, measured along the circumference of a track.)
The functioning of magnetooptical disk memories is based on the magnetooptic effect, which is characteristic of certain magnetic materials, in particular alloys that include a metal such as iron, cobalt or chromium and one of the metals of the heavy rare earth group such as terbium, gadolinium or dysprosium.
This effect involves the interaction of a rectilinear polarizer light with the magnetic state of the material. It can occur by transmission of the light through the material, in which case the magnetooptic effect is known as the Faraday effect, or by reflection on the recording material, in which case it is known as the Kerr effect.
The interaction of a rectilinear polarized light with the magnetic state of the material comprising the magnetic recording medium causes the rotation of the electrical field vector in the plane perpendicular to the direction of propagation of the light, or the rotation of the plane of polarization of the light, which includes both this electrical field vector and the direction of propagation.
If the Kerr effect is operative, it is observed that after the reflection of the incident beam of light on the magnetooptical material, the electrical field vector of the light undergoes a rotation, which is conventionally said to equal an angle -.theta. when the light beam encounters a negatively magnetized domain (by convention) and to equal an angle +.theta. when the beam encounters a positively magnetized domain (again by convention).
It is apparent that to read the data recorded on a magnetic medium for magnetooptical recording, it is sufficient to detect either the rotation of the electrical field vector or the rotation of the plane of polarization of the light. This is done by means of a light analyzer disposed such that if the light is reflected onto a negatively magnetized magnetic domain, a light of zero intensity is collected at its output, and if the light is reflected onto a positively magnetized magnetic domain, a light of non-zero intensity is collected. By placing one or more photoelectronic transducers, also known as photodetectors, behind the light analyzer, a zero voltage signal is received at the output of these detectors for a negatively magnetized magnetic domain, and a non-zero voltage signal is received for a positively magnetized magnetic domain.
A more detailed discussion of the manner in which the data in a magnetooptical memory are read is provided in U.S. Pat. No. 4,510,544.
Magnetooptical angles of rotation (.theta.) are generally small, and consequently the difference in light intensity between a light of zero intensity and a light of non-zero intensity at the output of the analyzer is very slight. As a result, the signal collected at the output of the photodetector is slight, and so the signal includes a relatively large proportion of noise.
To improve the signal-to-noise ratio in reading, it has been proposed in current practice to modulate the polarization of the beam of light aimed at the magnetooptical material, for example as described in the article, "Dynamic detection of Faraday rotation" by Safwat G. Zaky, in IEEE Transactions on Magnetics, September, 1972.
A beam of light is polarized by making the plane of its polarization oscillate by an angle .theta.'=a sin .omega. t about a mean position. At the output of the analyzer and of the photoelectronic detectors, a carrier beam of light is detected, the amplitude of which is proportional to the rotation .theta. of the plane of polarization of the light caused by its reflection onto the magnetic material comprising the recording medium. By disposing filtering means and amplifiers at the output of the photodetectors, the continuous levels are eliminated, thereby cutting off the noise they contain, that is, the noise due to the photodetectors, to fluctuations in the power of the laser emitter of the polarized light, to dust, to scratches on the polarizers and analyzers, and to defects that the beam of light may encounter, for example on the magnetooptical disk.
The advantages mentioned above supplement the known improvement in the signal-to-noise ratio attained by using a modulated signal.
Known devices for modulating the polarization of light are described, for example, in the thesis entitled "Modulation de polarisation avec des grenats magnetooptiques" [Polarization modulation with magnetooptical garnets"] by Ms. Pascale Valette, which was submitted for an advanced degree in materials science and defended in June 1984 at the University of Paris VI. One such device associates a periodic magnetic field generator having a pulsation .omega. with a magnetooptical garment that is transparent to light, the magnetization of which is parallel to its surface and the magnetooptical properties of which are based on the Faraday effect. The periodic field is sent to the garnet with an incidence .alpha. not equal to 90.degree..
When the magnetic field having the pulsation .omega. is sent to the magnetooptical garnet, the magnetization in the garnet changes direction periodically (it is understood that the intensity of the magnetic field is greater than the coercive field of the material comprising the magnetooptical garnet). It follows that the angle of rotation of the plane of polarization of the light (due to the Faraday effect) sent to the magnetooptical garnet simultaneously with the magnetic field will itself vary periodically, with a pulsation close to the pulsation .omega.. After having passed through the mangetooptical garnet, the beam of polarized light undergoes a rotation .theta.' due to the magnetooptical effect (equal to a sin .omega. t) of its plane of polarization with respect to the plane of polarization of the incident beam sent to the surface of the magnetooptical garnet.
The major disadvantage of this type of modulator device is that it can function only at low frequencies, and so to read the data in magnetooptical disk memories having the aforementioned high recording densities, the polarization of the light must be modulated with very high frequencies. In fact, the data reading output of magnetooptical memories of very high recording density is on the order of one to several tens of millions of data per second, which corresponds to reading frequencies of several tens of a megahertz. For the modulated beam of light to be a carrier, its frequency must be at least double the reading frequency, and hence must be on the order of several tens of a megahertz.
Yet the light polarization modulator devices of the type known in the art, as described above, cannot function at frequencies higher than a maximum of 100 kilohertz.